The present invention relates to the field of germicidal systems employing bacteria-destroying ultraviolet lights. In particular, the present invention relates to a system for producing an air flow through a baffled ultraviolet sterilization chamber, wherein the ultraviolet light intensity, the air residency time, and the air exchange rate for the air volume in a given space, are such that a percentage of tuberculosis bacteria are destroyed that effectively prevents transmission of such disease by airborne sputum.
Tuberculosis is the most common cause of death from infectious disease in the world today. It infects millions of people each year and causes hundreds of thousands of fatalities. The disease is particularly prevalent in less-industrialized countries where high population densities, poor sanitary conditions and a high percentage of individuals in poor health contribute to the spread of infectious diseases.
After a long period of declining rates of tuberculosis infection in the United Sates, it is believed that the infection rate is now increasing. The increasing rate is apparently due to a combination of factors. One factor is undoubtedly increased immigration from parts of the world with high rates of infection. For example, in the United States the case rate of tuberculosis per 100,000 population was 9.3 in 1985, resulting in over 22,000 cases and over 1,200 deaths. In Southeast Asia, both the case rate and the death rate are believed to be many times that, and immigrants from that part of the world now constitute 3 to 5% of new cases in the United States.
Another factor related to increased rates of tuberculosis infection appears to be the use of living quarters with high population densities and less-than-ideal sanitary conditions for persons in ill health who are susceptible to the disease. Such conditions are commonly found in shelters for the homeless, prisons and some nursing homes. Another important factor in the increased rate of infection is infections among patients with Acquired Immune Deficiency Syndrome (AIDS) and intravenous drug users.
Another reason for the recent increased incidence of tuberculosis is probably the failure of many medical professionals to diagnose and treat the disease early and properly. The relative rareness of the disease in the United States since the early epidemics resulted in an entire generation of health care workers without much experience in the disease. Further, diagnosing the disease is not always easy, for the symptoms are similar to the symptoms of many other disorders. Therefore, the disease is often misdiagnosed and mistreated, and the degree of infectiousness of the disease is underappreciated.
Even after it is recognized that a set of symptoms may indicate tuberculosis, the tests for the disease are somewhat imprecise and tend to require judgment by an experienced professional. For example, one diagnostic tool is chest x-rays which typically show apical-posterior segment cavitary changes in tuberculosis infected patients. However, in elderly individualsxe2x80x94who comprise a relatively large proportion of tuberculosis patientsxe2x80x94lobar or patchy lower-zone shadows may simulate bacterial or aspiration pneumonia. Also, x-rays in the elderly may mislead the physician by showing a solitary pulmonary nodule or a pleural effusion. Another important tuberculosis test is the tuberculosis skin test, but a major disadvantage to the tuberculosis skin test is that it generates a high number of both false-positive and false-negative results. The most precise test is microscopic examination of a sputum sample, but this test may require the use of at least three separate samples of sufficient volume, which may require gastric aspiration or bronchoscopy in patients with low sputum production.
The normal body reaction to infection by tuberculosis bacteria is to build a fibrous wall around each bacterium. Initially, a person may be unaware of any infection, but over a period of months or even years the infection produces inflammation and eventually destruction of tissue. The manifestations as the disease progresses generally include cough, fever, night sweats, hemoptysis, chest pain, weight loss and malaise. The usual treatment for tuberculosis is administration of drugs over a period of many months such as isoniazid, rifampin and pyrazinamide and ethambutol. Persons recently infected but with no active disease are usually given isoniazid preventive therapy, particularly if they have other risks such as malnutrition, gastrectomy, diabetes mellitus, pneumoconiosis, malignancy or if for some reason they have immunosuppression such as from corticosteroid therapy, renal impairment or HIV infection. In short, tuberculosis in a normal healthy patient is typically a disease that is curable by drugs, although the drug therapy is quite prolonged.
A serious concernxe2x80x94and yet another reason for the recent increase in tuberculosisxe2x80x94is the development of drug-resistant tuberculosis. It is estimated that at least 5% of new cases are resistant to the usual drug therapy, and that the percentage in some areas of the United States is as high as 20%. While non-drug-resistant tuberculosis is typically 99% curable in patients with normal immune responses, drug-resistant tuberculosis is only about 50-60% curable. A related concern is drug therapy on non-drug-resistant tuberculosis for patients who are intolerant of the drugs. In those cases, drug therapy is complicated because the drug is effective against the infection but has serious adverse effects on the patient such as hepatitis or serious rashes.
Another concern is raised by the increasing incidence of non-tuberculosis mycobacterial pulmonary infections. Many such infections produce symptoms similar to those of tuberculosis infections, but may be more difficult to identify and treat. Moreover, they may be transmitted through the same means as tuberculosis and tend to infect the same types of susceptible individuals.
The transmission of the tuberculosis bacteria is accomplished almost exclusively by infected individuals expectorating microdroplets of bacteria-containing sputum by coughing or sneezing. These microdroplets are suspended in the air and are inhaled by other individuals in the vicinity. The bacteria typically lodges in the lower lung where it proliferates, and may be disseminated to other organs as well. The microdroplets of sputum which contain the bacteria may be very smallxe2x80x94on the order of 0.01 microns. In fact, it appears that the smallest droplets are the most effective in communicating the disease since the smallest droplets stay airborne indefinitely and are easily inhaled to the lower lung where they are not readily removed. Studies have shown that aerosol droplets on the order of 1-5 microns are highly effective vehicles for transmitting the disease.
One controversial approach to combatting the disease has been the use of vaccines. However, the efficacy of tuberculosis vaccines is debatable. Even the trials which seemed to show some efficacy have shown less efficacy among adults than among infants and children. An additional objection to widespread vaccinations is that by inducing tuberculin reactivity in the population they would confound the detection and measurement of infections through the use of skin tests, since skin tests in vaccinated individuals would presumably result in a false-positives. This would severely curtail the practice of preventive drug therapy among infected patients who have not yet developed outward symptoms.
The airborne aspect of the disease has led toward systems for preventing the transmission of the disease which focus on filtration and sterilizing devices. One approach is the use of masks. Simple surgical masks are thought to be insufficient in view of the very small size of the sputum microdroplets which are effective in communicating the bacteria. Instead, disposable particulate respirators are recommended. The use of masks is fraught with practical difficulties; they are physically uncomfortable, they impair breathing (which is already impaired for many patients), and they disrupt speaking. To be effective at all, it would probably be necessary for the masks to be worn not just by the patients, but also by noninfected individuals. In view of the long distances that airborne microdroplets containing viable bacteria can travel, it would be necessary for the masks to be worn by noninfected individuals throughout the general vicinity of a patient and not just those in the immediate presence of a patient. Moreover, it is not known for certain whether the use of masks would actually be effective even if the practical problems were tolerated or overcome.
Another preventive measure which relies on the airborne aspect of the bacteria is the use of modified ventilation systems. It is currently recommended that facilities used for tuberculosis patients undergo certain minimum air exchange rates, under the theory that dilution of infectious air with clean air will reduce the concentration of bacteria and hence the likelihood of transmission of the disease. While this approach is theoretically sound, it is problematic in implementation. Modern buildings are normally designed with fixed ventilation systems which are not easily modified to produce the requisite air exchange rate. Even if they are suitably modified, they may be rendered ineffective by an open door or by shifting air-flow patterns. A high air exchange rate also increases cooling and heating costs. Finally, there is the issue of the ultimate disposition of the contaminated air that is removed, and whether it is appropriate to simply release it outside the facility.
Another approach to reducing the transmission of the disease is the use of high-efficiency filtration systems. For such a system to be effective, however, it must employ a very dense filter to trap very small particles. This entails a powerful fan, high energy usage, loud noise, and meticulous installation and maintenance. There is also concern that the filters and the rest of the air-flow path may themselves become sites of bacteria colonization.
Yet another approach to reducing the transmission of the tuberculosis bacterial employs ultraviolet light as a germicide. It was discovered some time ago that airborne bacteria are susceptible to ultraviolet light in wavelengths of about 254 nm. Wells S. F., On Air-Borne Infection: II-Droplets and Droplet Nuclei, Am. J. Hyg. 1934 20: 611-8; Wells W. F., Fair G. M., Viability of E. Coli Exposed to Ultraviolet Radiation in Air, Science 1935; 82:280-1. That finding led to the development of systems using ultraviolet light as a germicide against airborne bacteria such as measles and tuberculosis. However, interest in such systems diminished when later investigators were unable to obtain the desired efficacy. Also contributing to the diminished interest in such systems was the recognition that ultraviolet lights produced harmful ozone and also produced skin and eye irritation. With the development of streptomycin and chemotherapy for tuberculosis treatment, the belief became prevalent that tuberculosis would be eradicated and that preventive systems would be unnecessary.
The systems that were developed using ultraviolet light as a germicide against tuberculosis were imprecise, marginally effective, and perhaps dangerous. The most common system simply employed ultraviolet lights mounted on or suspended from a wall or ceiling of a room. For example, a system employing lights suspended from the ceiling is described in some detail in Riley, R. Z., Knight, M. and Middlebrook, G., Ultraviolet Susceptibility of BCG and Virulor Tubercle Bacilli, Am. Rev. of Resp. Dis., 1976, 113:413. The problems in such a system are numerous. It relies completely on normal air circulation in the room where it is installed to bring the bacteria within range of the ultraviolet light. The normal circulation in a room may be too low for the ultraviolet light to destroy a necessary proportion of bacteria, or the normal circulation may be high enough but of a pattern that does not bring the airflow past the ultraviolet light. Moreover, there is no single test to determine whether the circulation rate and patterns are adequate or not for a given installation. Further, such systems quickly become contaminated by dust on the light bulbs which diminishes their effectiveness. From a safety standpoint, one of the greatest concerns is that the simple light shields used with such systems allow light to be reflected off the walls and ceiling and onto the skin and eyes of the occupants. The degree of danger associated with the indirect ultraviolet irradiation is disputed, but there is undoubtedly at least some danger if the period of exposure is prolonged. In explaining the necessary safety precautions, Riley, R. L. and Nordell, E.A., Clearing the Air, The Theory and Application of Ultraviolet Air Disinfection, Am. Rev. Respir. Dis. 1989 139:1286, stated:
Does germicide UV cause inflammation of skin and eyes? It can, but the standard set by the National Institute of Occupational Safety and Health (NIOSH) is very conservative. Overhead installations must be inspected for xe2x80x98hot spotsxe2x80x99 (greater than 0.2 uW/cm2) with a sensitive UV meter. Installers should anticipate readjusting fixture height up or down based on meter readings. Baffles designed to prevent direct eye contact will also need adjustment after the initial installation. Excessively reflective surfaces about fixtures may contribute to excess radiation, but this can be reduced with nonreflective paint or by spraying the surface with stove black. If the intensity of UV does not exceed 0.2 uW/cm2, the likelihood of skin or eye irritation is minimal during an 8-h exposure. Persons with especially sensitive skin, with systemic lupus erythematous, for example, may need to avoid exposure or take measures to protect their skin.
This illustrates some of the difficulties and dangers of employing ultraviolet lights behind a simple light shield; the light may generate dangerous and unpredictable xe2x80x9chot spotsxe2x80x9d, it is not appropriate for those with sensitive skin or eyes, and it requires careful consideration of the placement and the orientation and reflectivity of the surrounding surfaces. Finally, even if all those precautions are observed, the quote only indicates that skin and eye irritation is xe2x80x9cminimalxe2x80x9d rather than nonexistent and only for exposure periods of 8 hours. Of course, for the system to be effective against transmission of airborne disease in, for example, a patient room, it would have to operate continuously and not just for 8 hour periods. The article goes on to acknowledge that:
UV or disinfection that is inappropriately applied, poorly planned, or carelessly used may be ineffective, dangerous, and falsely reassuring. The guidelines and precautions listed above are not intended to enable a would-be user of UV to plan, purchase, install, or check the adequacy of a UV installation. Detailed instructions for UV installers have been published. However, there is currently little commercial interest in UV for air disinfection and, therefore, little expert guidance for comprehensive planning and installation. Renewed consumer interest may stimulate the UV industry to correct this deficiency.
Notwithstanding the uncertainly expressed in the Riley and Nordell article regarding the dangers of ultraviolet radiation, that article is actually more cognizant of those dangers than much of the other literature on the subject. For example, the article by Riley, Knight and Middlebrook, supra, does not even mention the dangers to the skin and eyes of ultraviolet radiation, or any precautions that should be taken to minimize those dangers.
There are number of ultraviolet germicidal systems that have been patented, but as in the case of the scientific literature mentioned above, those patents teach little about the dangers of ultraviolet radiation and how to effectively minimize the dangers, or how to position and operate the devices to achieve the requisite bacterial kill rate to prevent transmission of disease.
For example, U.S. Pat. No. 3,975,790 by Patterson is for an ultraviolet lamp fixture used in combination with a conventional commercial vacuum cleaner, and U.S. Pat. No. 4,087,925 by Bienek is for a sterilizing hand dryer, in which ultraviolet lights are positioned within the housing of a blower that is used to dry wet hands, where the blower is of the type commonly used in commercial restrooms. The devices of Patterson and Bienek seem to include little or nothing for light baffling to prevent leakage of allowable light to outside the housing, and the patents teach nothing about optimal flow rates, air-exchange rates or other information for the effective use of the machines. The devices are obviously intended as general, and only partially effective, sterilizing tools rather than as comprehensive and predictably effective systems.
Another patent, U.S. Pat. No. 4,210,429 by Golstein, employs a xe2x80x9csquirrel-cagexe2x80x9d type blower which draws air into a housing through a air intake filter, through the blower, and through a sterilization chamber containing ultraviolet lights. The air leaves the sterilization chamber, passes through a second filter and a charcoal filter and finally exits through an outlet. The specification indicates that the purpose of the device is to remove xe2x80x9cpollens, lung damaging dust, smoke, bacteria and any one of a number of other irritants and micro-organismsxe2x80x9d and that it does so for xe2x80x9cparticles down to 0.3 microns in size with an efficiency of 99.9%xe2x80x9d. The device is characterized as an xe2x80x9cair purifierxe2x80x9d rather than as a germicidal device; the use of three distinct filters including a very fine filter for removing extremely small particles, a charcoal filter for removing odors and a pre-filter for removing particles, is distinguishable in design and function from the present invention. This extensive filtration would require a high-capacity blower to achieve any effective air exchange rate. The device is not specifically designed for destroying the tuberculosis bacteria or any other specific bacteria, although it would obviously be effective in doing so to some extent. Therefore, the patent teaches nothing about the use of the device for that purpose or the optimal flow rates or positioning of the device for that purpose.
U.S. Pat. No. 5,074,894 by Nelson is for a hospital room to quarantine patients with tuberculosis or other respiratory diseases caused by airborne pathogens. Although one embodiment of the system includes an air circulation circuit with ultraviolet lights, the patent is directed primarily toward negative pressure and filtering aspects utilizing high-efficiency particulate air filters.
Other patents describing the use of ultraviolet light as a germicide against airborne bacteria include, U.S. Pat. Nos. 4,448,750 by Fuesting, 4,896,042 by Humphreys, 4,990,311 by Hirai and 4,047,072 by Wertz. Patents directed toward the use of ultraviolet light as a germicide against bacteria in water or other liquids include U.S. Pat. Nos. 4,400,270 by Hillman, 4,482,809 by Maarschalkerweerk, 5,102,450 by Stanley and 5,124,131 by Wekhof.
The present invention is an apparatus and process for destroying airborne pathogenic bacteria such as the tuberculosis bacteria. Ultraviolet lights of a sufficient intensity are positioned within a sterilization chamber where they irradiate an air stream containing the bacteria, typically in the form of suspended microdroplets of sputum. The sterilization chamber has an exit and an entrance, and a blower is positioned preferably at the exit to draw air into the entrance and through the sterilization chamber and out the exit. The air passing through the sterilization chamber is virtually completely sterilized of viable tuberculosis bacteria by the chosen dosimetry of the system, which is achieved by appropriately sizing the sterilization chamber employing ultraviolet lights of the correct intensity, and utilizing the right air flow rate through the blower.
The sterilization chamber includes a filter on the intake side to filter out large particles such as dust, in order to minimize the contamination of the ultraviolet light bulbs. The filter is deliberately designed not to intercept small particles such as microdroplets, since the filter could then become a bacteria colony. The use of a low density filter also minimizes the resistance to air flow, thereby allowing the use of a smaller, more efficient and quieter blower. With the exception of this intake filter for removing large particulates, the apparatus preferably does not include any devices that would intercept and retain microdroplets or other small particles in a way that resists the air flow and poses the possibility of becoming a bacteria colonization site; the small particulates and microdroplets with destroyed bacteria simply pass through the apparatus and are expelled back into the environment.
Both the air intake and exhaust to the sterilization chamber are baffled so that ultraviolet light must reflect off multiple surfaces before exiting the sterilization chamber. The interior surfaces of the baffles may be light-absorptive to minimize their reflectivity and further lessen the possibility of ultraviolet light leaking from the sterilization chamber into the environment.
The apparatus is used in a space having a volume of air that results in an air exchange rate of preferably 12-15 air exchanges per hour. At that air exchange rate, it has been determined that a sufficient volume of air will circulate through the apparatus and will prevent any air stagnation in the room, that a high enough percentage of tuberculosis bacteria will be destroyed before they are inhaled by persons in the room to prevent transmission of the disease. The apparatus is preferably positioned such that the air intake is approximately at the head level of a seated patient, so that the air intake draws the air volume containing the highest level of bacteria-containing sputum microdroplets expectorated from the patient""s pulmonary system.